Japanese Patent Unexamined Publication No. H05-191312 discloses a conventional radio-frequency receiver. As shown in FIG. 7, conventional radio-frequency receiver 201 contains radio-frequency receiving circuits 202a and 202b that are identical in construction. Radio-frequency circuit 202a (202b) contains input terminal 204a (204b) that receives digital broadcasting signals from antenna 203a (203b), output terminal 205a (205b) that carries output signals and PLL-control terminal 206a (206b) that carries PLL-control signals. Radio-frequency receiving circuits 202a and 202b are accommodated in a same housing.
Besides, radio-frequency receiving circuit 202a (202b) contains mixer 207a (207b) and oscillation circuit 209a (209b). Mixer 207a (207b) receives a signal from input terminal 204a (204b) and sends it to output terminal 205a (205b). Mixer 207a (207b) also receives an oscillation signal from oscillation circuit 209a (209b) via selective switching section 208a (208b).
Such structured radio-frequency receiver 201 is usually battery-operated, for example, at a low voltage of 3V. It is therefore difficult to supply high tuning voltage to variable capacitance diodes that constitute oscillation circuits 209a and 209b in terms of decreasing power consumption and noise.
To address the problem, each of oscillation circuits 209a and 209b contains four connected units of an oscillator and a tuning circuit: three units for selectively receiving all channels (hereinafter abbreviated to ch) in the UHF (ultra high frequency) band; and one unit for receiving 7-ch in the VHF (very high frequency) band.
That is, conventional radio-frequency receiver 201, since having two radio-frequency receiving circuits 202a and 202b, needs eight units of the oscillator and the tuning circuit in total.
The tuning circuit is formed of a capacitor and an inductor connected in parallel; the inductor has a large footprint in the circuit. This has been an obstacle to reduction in size of an integrated circuit; accordingly, an obstacle to reduction in size of a radio-frequency receiver.